1. Field of the Invention
The present invention relates to analog and digital circuits. In particular, the present invention relates to voltage reference circuits for providing stable reference voltages for analog and digital applications.
2. Background of the Prior Art and Related Information
In electronic design, stable voltage references are needed for a wide variety of analog and digital applications. Such applications include voltage regulators, current supplies for ECL logic, etc. For lower voltages, however, and also where stability over a large temperature range is required, providing a precision reference voltage poses considerable problems. In particular, a voltage reference having good stability over a wide temperature range, such as the standard military specification temperature range of -55.degree. C. to +125.degree. C., is very difficult to achieve in a commercially practical implementation.
One conventional approach to providing a voltage reference has been to use temperature compensated zener diodes. Since the breakdown voltage of a zener diode is about 6 volts, however, this provides a lower limit on the input voltage employed in a voltage regulator circuit. Other disadvantages are also associated with zener diode voltage references, such as stability problems, process control problems and noise introduced into the circuit.
In another approach, the bandgap voltage of silicon is employed as an internal reference to provide a regulated output voltage. This approach overcomes many of the limitations of zener diode voltage references such as long-term stability errors and incompatibility with low voltage supplies. One such conventional bandgap voltage reference is disclosed in R. Widlar, New Developments in IC Voltage Regulators, IEEE J. Solid-State Circuits, Vol. SC-6 (February 1971), and is illustrated generally in FIG. 1. In this approach, a relatively stable voltage is established by adding together two scaled voltages having positive and negative temperature coefficients, respectively. The positive temperature coefficient is provided by the difference between the base-emitter voltages of two bipolar transistors Q1 and Q2 operating at different emitter current densities (referring to FIG. 1). Since these two transistors are operated at different current densities, a differential in the emitter-base voltages of the two devices is created and appears across R3. The negative temperature coefficient is that of the base-emitter junction of transistor Q3. Thus the basic bandgap cell requires three transistors, Q1, Q2 and Q3 to achieve the offsetting temperature coefficients. It can be shown that, for theoretically perfect device operation, if the sum of the initial base-emitter voltage of Q1 and the base-emitter voltage differential of the two transistors Q1 and Q2 is made equal to the extrapolated energy bandgap voltage, which is +1.205 V for silicon at T=0.degree. K., then the resultant temperature coefficient equals zero. (The detailed derivation of this result may be found in the above-noted Widlar reference.)
Another example of a bandgap voltage reference is described in A.P. Brokaw, A Single Three-Terminal IC Bandgap Reference, IEEE J. Solid-State Circuits, Vol. SC-9, No. 6 (December 1974). This type of bandgap voltage reference circuit is illustrated generally in FIG. 2. This circuit employs a variation of the Widlar bandgap reference circuit, wherein two reference transistors Q1 and Q2 are implemented with a collector-current sensing amplifier A to establish the bandgap voltage. The emitter current densities of Q1 and Q2 are adjusted by varying their relative size. Amplifier A, in conjunction with the collector load resistors, senses the collector currents of the reference transistors and forces them to be equal. Alternatively, a current mirror configuration is employed to sense the collector currents of Q1 and Q2. By adjusting R1 and R2 the differential base-emitter voltage of Q1 and Q2 can be used to provide a positive temperature coefficient term which compensates the negative temperature coefficient of the base-emitter voltage of Q1. This compensated voltage appears as V.sub.OUT.
These above-described bandgap voltage references, using two or three-transistor bandgap cells, allow operation with very low voltage sources, as compared to the earlier 6 V limitation of avalanche diodes, as well as providing greater stability.
Although these conventional bandgap voltage references provide several advantages for voltage reference design, for practical non-ideal bipolar transistors, perfect temperature compensation is not provided. For both the above-mentioned conventional bandgap voltage reference circuits, the actual temperature characteristic is a parabolic temperature curve due to nonlinearities of the temperature behavior of the transistors forming the bandgap cell, as well as to nonlinearity of the circuit resistance temperature coefficient. Such a parabolic temperature dependent curve is illustrated in FIG. 3 for the standard military specification temperature range of -55.degree. C. to +125.degree. C. As shown in FIG. 3, the bandgap-stabilized reference voltage gradually decreases both above and below the nominal compensation temperature (typically room temperature) thereby causing a parabolic temperature curve. This curvature puts a limit on the achievable accuracy of the reference voltage over the desired operating range in conventional bandgap references. Thus, even though the voltage at room temperature can be trimmed to accuracies within approximately 0.5%, over the typical -55.degree. C. to +125.degree. C. temperature range, the curvature of the temperature coefficient of the base-emitter reference voltage limits accuracy to approximately 2% for production quantities.
One approach to compensating for such temperature variations in bandgap voltage references is described in G. Meijer, P. Schmale, and K. Van Zalinge, A New Curvature-Corrected Bandgap Reference, IEEE J. Solid-State Circuits, Vol. SC-17, No. 6 (December 1982). The Meijer et al. article deals with compensation for the thermal nonlinearity of the base-emitter voltage. The article discusses thermal compensation by adding together the correction voltage that is proportional to the absolute temperature squared with the same voltage that is not squared, theoretically providing correction in curvature of the temperature characteristic of the bandgap voltage reference. While in theory, such an approach to compensation may be used, for practical applications, such corrections are extremely difficult to implement. In particular, it is difficult to trim the circuit since all of the temperature coefficients must be extremely precise or else the temperature characteristic of the reference circuit could have even greater nonlinearity. More specifically, a lack of reproducibility arises due to the additional voltage trimming required by adding the squared voltage constant. Additionally, a large number of transistors is necessary to implement the curvature-corrected reference discussed by Meijer et al. Furthermore, only the base-emitter voltage is compensated and the temperature coefficient of the circuit resistance is improperly disregarded, since this resistance value also determines the reference voltage curvature.
Accordingly, a need presently exists for a voltage reference circuit having high precision over a wide temperature range, such as the mil. spec. range -55.degree. C. to +125.degree. C., which may be implemented in a manner readily achieved for production quantities.